A work piece conveyor system having a plurality of load carriers and a work piece conveying apparatus is known from EP 0 163 617 A2. The work piece conveying apparatus includes a conveyor chain which runs within a recessed track of a guiding rail, and which is supported at the bottom of the recessed track of the guiding rail. Each of the load carriers comprises a front guiding foot and a rear guiding foot, which both stand on the conveyor chain within the recessed track so that the conveyor chain supports the weight of the load carriers and conveys the load carriers via transfer of friction forces. Side walls of the recessed track of the guiding rail guide both the conveyor chain and the guiding feet of the load carriers standing on the conveyor chain, and thus also the load carriers themselves. In processing stations the load carriers are stopped for processing work pieces loaded onto the load carriers. A number of load carriers may be queuing in front of such a processing station. The conveyer chain within the recessed track goes on running beneath the guiding feet of the load carriers, as the conveyor chain is provided for a plurality of processing stations, i.e. for transporting all work pieces on all load carriers which are presently not waiting in front of a processing station over the entire length of the conveyor chain. To reduce the force applied to a processing station by a plurality of load carriers queuing in front of the processing station and standing on the conveyor chain running beneath them, it is known from EP 0 163 617 A2 to provide ring-shaped buffers made of elastic material between the single load carriers. The ring-shaped buffers are deformed by the guiding feet bumping onto each other in such a way that the buffers lean against the side walls of the recessed track of the guiding rail and that they, thus, provide for a support of the load carriers at the guiding rail. The transfer of friction forces from the conveyor chain onto the guiding feet of the load carriers however, is not interrupted here. The forces applied by the load carriers are also still the same, but they are supported over a longer area of the guiding rail and they do not solely rest on the respective processing station in front of which the load carriers are queuing. The friction forces applied by the conveyor chain onto the single load carriers are considerable as the load carriers, in vertical direction, are only supported via their guiding feet by the conveyor chain. This results in high forces normal to the contacting surface between the guiding feet and the conveyor chain, particularly with heavy work pieces on the load carriers.
In another known work piece conveyor system having a plurality of load carriers and the same work piece conveying apparatus as described before, each load carrier has one additional support roller additionally supporting the load carrier at the guiding rail. The support roller rotating about a horizontal rotation axis rolls on a supporting surface provided by the guiding chain on one side of the recessed track. With regard to the direction of the conveyor chain, the support roller is arranged between the two guiding feet. Thus, there is a three point support of each load carrier at the guiding rail. In this three point support the weight forces onto the guiding feet to be supported by the conveyor chain are reduced, and thus the friction forces transferred from the conveyor chain onto the load carriers may also be reduced. Further, the load carriers are stabilized about the longitudinal axis of the conveyor chain so that the center of weight of the load carriers with and without work piece no longer needs to be exactly located above the conveyor chain but may also be laterally offset. However, the laterally offset support roller results into moments of force about a vertical axis onto the load carriers. These moments of force are to be supported by guiding the guiding feet within the recessed track of the guiding rail. To reduce these moments of force, the supporting roll is swiveling with regard to the remainder of the load carrier about a vertical swiveling axis running at a distance to its horizontal rotation axis. For reducing the force applied to a processing station by a plurality of load carriers queuing in front of the processing station, these known load carriers comprise a device which raises the two guiding feet of each load carrier in the queue away from the conveyor chain which has been in contact with both guiding feet of each load carrier up to then. To this end a front and a rear lever element are provided at each load carrier, each of which lever elements is swiveling about a horizontal swiveling axis running across the guiding rail. With a single load carrier the lever elements have inclined orientations and do not yet contact the guiding rail. With queuing load carriers contacting each other via their lever elements, the lever elements are pressed downwards swiveling about their swiveling axes, until they touch the guiding rail besides the recessed track and raise the load carriers until their guiding feet are lifted away from the conveyor chain. As soon as the guiding feet are no longer in contact with the conveyor chain, no further transfer of friction forces from the conveyor chain onto the load carrier takes place. However, the lever arms only arranged on one side of the recessed track also result into moments of force about onto the load carriers a vertical axis, as soon as the lever arms lean against the supporting surface, even if rollers are arranged at their ends which reduce the friction between the lever arms and the supporting surface. To lift both guiding feet away from the conveyor chain both the front and the rear lever element has to be pressed down, which only occurs with the two lever arms of a load carriers, if these lever arms are pressed together by the lever arms of a preceding and a succeeding load carrier within the same queue. I. e. with a load carrier bumping onto a queue of load carriers only the front guiding foot is lifted away from the conveyor chain via the front lever element, and the conveyor chain still applies friction forces onto the rear guiding foot. Further, it takes some efforts to provide the lever arms at the load carriers and to care for their full function.
Thus, there is still a need for a work piece conveyor system in which the load carriers are of simple construction, in which no moments of force are applied to the load carriers about a vertical axis, and in which nevertheless the forces onto a processing station applied by a plurality of load carriers queuing in front of the processing stations can effectively be reduces.